Car turns off while driving, loss of power?

I have a ford Fiesta ztec 2006 hatch, it has less than 40,000 km while driving the car just turns off. Monitered the speeds, weather, the fuel ( makes no difference ). Losses power at anytime. I have taken it back to Ford on 3 occassions for the whole day and they have no idea. What do you think the problem is ?. Currently at another mechanics.